1. Field of the Invention
The invention relates generally to optical communications systems. In particular, the invention relates to optical code division multiple access.
2. Background Art
Communication networks based on optical fiber as the transmission medium have the potential of extremely wide bandwidth measured in many terahertz. However, transmitters and receivers are partially based on electronics and thus limited at the present time to about 10 gigabits per second (Gb/s). Further substantial increases in speed are unlikely. Even the 10 Gb/s equipment is expensive, large, and power intensive. Wavelength division multiplexing (WDM) has been utilized to increase the data bandwidth while using electronics of limited speed. In WDM, multiple optical carriers of wavelengths differing by about a nanometer or less are impressed with separate data signal and are then multiplexed onto a single optical fiber. However, WDM does not fully exploit the available bandwidth, at least partially because of the necessity of completely separating the spectrally different WDM components at the receiver or at any switching points in the network. Conventional WDM and time division multiplexing (TDM) schemes require relatively complex and expensive WDM or TDM switches or multiplexers at the central nodes to distribute traffic in formats limited by the WDM and TDM schemes to other nodes. Such schemes are not attractive for access networks needing equipment of low cost and high bandwidth.
Optical code division multiple access (O-CDMA) has been proposed. However, these have often been derived from RF CDMA architectures in which a series of pulses are amplitude modulated with a code. For optical systems this amounts to power modulation. Such an approach does not circumvent the need to operate the electronics at very high bit rates. Furthermore, the binary coding of pulses allows a relatively small number of uses to be transmitting with different codes without interference. Other O-CDMA systems have included a 2-D grid of wavelength and time. These are considered too cumbersome.
O-CDMA is most easily applied to local area networks to which a large number of user terminals are attached. However, cost is an issue so that it is desired that the user terminals operate at significantly lower bit rates than the aggregate bit rate of the network.
Long-haul optical communications networks typically rely upon a shared network, for example, a public telephone network. Since public networks are subject to eavesdropping, the network security needs to be improved for users sending highly sensitive information, such as the military.